But blood from camels in Oman and Spain had antibodies against MERS or something closely related to it, Reusken and colleagues reported online in Lancet Infectious Diseases.

The finding, combined with anecdotal reports that some MERS patients had been in contact with domestic camels, suggests that dromedaries might be a source of the virus, the investigators said.

"As new human cases of MERS-CoV continue to emerge, without any clues about the sources of infection except for people who caught it from other patients, these new results suggest that dromedary camels may be one reservoir of the virus that is causing MERS-CoV in humans," Reusken said in a statement.

One implication -- pending further research to support the finding -- is that clinicians treating people with MERS should ask about contact with camels or products made from them, Reusken and colleagues concluded.

The study is the first report of a potential reservoir for the virus since the outbreak was recognized nearly a year ago, commented Emmie de Wit, PhD, and Vincent Munster, PhD, both of the National Institute of Allergy and Infectious Diseases labs in Hamilton, Mont.

It is still not clear whether the animals were infected with MERS or a related virus, they noted in an accompanying commentary, but the study is an "important step" forward in understanding how the virus emerged.

Reusken and colleagues studied blood samples from 50 camels in the Middle East state of Oman and 105 from the Spanish Canary Islands, as well as from cattle, sheep, goats, and other camelid species in Europe and Chile.

They looked for antibodies to MERS, as well as antibodies that reacted to the SARS coronavirus, and another strain of coronavirus, dubbed HCoV-OC43, that can also infect humans.

None of the other animals had antibodies specific to MERS, but all 50 of the Omani camels and 15 of the 105 Spanish camels did, Reusken and colleagues reported.

"The best way to explain this is that there is a MERS-CoV-like virus circulating in dromedary camels, but that the behavior of this virus in the Middle East is somehow different to that in Spain," Reusken said.

"Dromedary camels are a popular animal species in the Middle East, where they are used for racing, and also for meat and milk," she added, "so there are different types of contact of humans with these animals that could lead to transmission of a virus."

She and colleagues called for "well-designed animal studies in the Middle East" that would pin down the virus causing the antibody reactions and compare it with the virus isolated from human cases.

"This will need to be done not only through veterinary studies, but also by careful follow-up of new human patients, ensuring that as much information as possible is gathered about patients' contacts with animals and animal products, such as camel milk," she said.

As part of surveillance of some 3,000 close contacts of patients, the Saudi health ministry identified seven health care workers whose nasopharyngeal swabs were positive by polymerase chain reaction for MERS, according to Ziad Memish, MD, Saudi Arabia's Deputy Minister for Public Health, and colleagues.

Although most confirmed cases of MERS have involved serious illness, the finding demonstrates there is a spectrum of illness and highlights an "urgent need" to develop rapid diagnostic tests and to pin down the clinical range of disease, Memish and colleagues argued.

The study by Reusken's group had support from the European Union, the European Centre For Disease Prevention and Control, and Deutsche Forschungsgemeinschaft. Several authors reported having a patent pending for a MERS-CoV receptor. Reusken and the other authors declared they had no conflicts of interest.

The editorial authors had support from the National Institute of Allergy and Infectious Diseases. They are employees of the agency.

The report by Memish's group was supported by the Saudi Arabian health ministry. Authors are employees of the agency.

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner